<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <input type="text" id="inputA" />
    <div id="box">box</div>
    <script>
      const inputA = document.getElementById("inputA");
      const box = document.getElementById("box");

      Element.prototype.add = function (a, b) {
        console.log(a + b);
      };
      inputA.username = "jack";
      inputA.age = 20;
      inputA.say = function () {
        console.log(this.username);
      };

      inputA.add(1, 2);
      box.add(1, 2);
    </script>
  </body>
</html>
